A liquid crystal display indispensably requires polarizing elements disposed on both sides of a liquid crystal cell because of an image forming method adopted therein and generally polarizing plates are adhered. Besides, on a liquid crystal panel, various kinds of optical elements have been used in addition to a polarizing plate in order to improve a display quality of a display. For example, there have been used a retardation plate for coloration prevention, a viewing angle expanding film for improving a viewing angle of a liquid crystal display and a brightness enhancement film for raising a contrast of a display. The films each are collectively referred to an optical film.
A pressure-sensitive adhesive is usually employed in adhering an optical film described above to a liquid crystal cell. An optical film and a liquid crystal cell or optical films are usually adhered to each other using a pressure-sensitive adhesive therebetween in order to reduce a light loss. In such cases, a pressure-sensitive adhesive attached optical film in which a pressure-sensitive adhesive is provided in advance on one surface of an optical film as a pressure-sensitive adhesive layer is generally used because of a merit such as that no necessity arises for a drying step of fix the optical film.
Acrylic pressure-sensitive adhesives including an acrylic polymer as a base polymer are frequently used as pressure-sensitive adhesives of the pressure-sensitive adhesive attached optical films, because of their good adhesive properties, transparency and so on. Methods for crosslinking acrylic pressure-sensitive adhesives often use isocyanate crosslinking agents, and a coupling with the copolymerized functional monomers of the acrylic polymer is generally used in such methods.
Such optical films are adhered to liquid crystal cells to form liquid crystal panels, which are used and incorporated into liquid crystal displays. Liquid crystal displays have been used at first for calculators and later for watches, televisions, monitors and so on. Liquid crystal displays are placed under various conditions such as hot conditions and humid conditions and thus required to have high durability such that display quality degradation can be prevented.
When liquid crystal displays are placed under hot or humid conditions, however, display unevenness can sometimes occur in the peripheral portion of the liquid crystal panel to cause display defects. The display unevenness in the peripheral portion can be significantly found, particularly when an optical film in which a viewing angle expanding film is placed on a polarization plate is used.
In order to reduce the display unevenness in the peripheral portion, it is proposed that a pressure-sensitive adhesive composition containing a plasticizer or an oligomer component should be used for the pressure-sensitive adhesive attached optical film (see Patent Literatures 1 and 2 below). However, these pressure-sensitive adhesive compositions have a problem in which the additive such as the plasticizer or the oligomer component can be precipitated to cause defects in appearance or degradation of the pressure-sensitive adhesive in a long-time heating test.
Patent Literature 1: JP-A No. 09-87593
Patent Literature 2: JP-A No. 10-279907